Apparatus and method for synthesizing vertically aligned carbon nanotubes

ABSTRACT

Methods and devices to synthesize vertically aligned carbon nanotube (VACNT) arrays directly on a catalytic conductive substrate without addition of an external metallic catalyst layer and without any pretreatment to the substrate surface using a plasma enhanced chemical vapor deposition (PECVD) method are provided. A method comprises providing a catalytic conductive substrate, that has not been pretreated through a plasma enhanced chemical vapor deposition (PECVD) method or other methods, to a PECVD device, etching the catalytic conductive substrate to form catalytically active nano-features on the surface of the catalytic conductive substrate, and growing vertically aligned carbon nanotubes on the surface of the catalytic conductive substrate, without an external metallic catalyst layer, by providing a carbon source gas to the catalytic conductive substrate.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. applicationSer. No. 16/023,038, filed Jun. 29, 2018, the disclosure of which ishereby incorporated by reference in its entirety, including all figures,tables, and drawings.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Grant Nos.DMR0548061 and DMR1506640 awarded by the National Science Foundation.The government has certain rights in the invention.

BACKGROUND

Since the discovery of carbon nanotubes (CNTs), time, money, and effortshave been expended to exploit their properties towards developing newdevices and improving current technologies. Due to their unique physicalstructures, including one dimensionality, high surface area, high aspectratio, and exceptional electronic, mechanical and thermal properties,CNTs have shown potential for a number of different applications

Plasma enhanced chemical vapor deposition (PECVD) has been used tosynthesize vertically aligned CNT (VACNT) arrays at lower temperatures.However, many PECVD techniques are complex and expensive and limited toresearch laboratory scale which make them unsuitable for the large-scaleproduction. There is also the possibility of having a weak attachment ofcatalyst nanoparticles to the substrate, which helps cause degradationof VACNT composites over time. Hence, for the applications like fieldemission, electrodes and sensors, it is desirable to grow VACNTsdirectly on a conducting catalytic substrate without addition of anexternal catalyst layer.

BRIEF SUMMARY

Embodiments of the subject invention provide methods and apparatuses forthe synthesis of vertically aligned carbon nanotube (VACNT) arraysdirectly on a catalytic conductive substrate without the addition of anexternal metallic catalyst layer and without any pretreatment to thesubstrate surface by using a plasma enhanced chemical vapor deposition(PECVD) method or other method. The VACNTs have uniform length, goodalignment, and uniform coverage over the catalytic conductive substratesurface. A study of surface morphology of the catalytic conductivesubstrate prior to the growth of the VACNTs, as revealed by atomic forcemicroscopy (AFM) and scanning electron microscope (SEM) images,underlines the occurrence of important surface evolution due to rampingthe temperature in the presence of an etching gas to form uniformnano-hills, which play a role in VACNT nucleation and the growthprocess.

Transmission electron microscope (TEM) analysis and energy-dispersiveX-ray spectroscopy (EDS) report show that the particles at the tip ofthe VACNTs are Fe crystal. Although other transition metals (Ni, Co, Mn)were also present at the catalyst nano-hills, only Fe allows thedissolution and precipitation of carbon and lifts off during the growthprocess. In addition, no trace of oxygen or carbide was found in thecatalyst tip particle indicating the purity of single crystal Fe. Hence,as-synthesized VACNTs with pure Fe at the tip are useful in the fieldsof drug delivery and field emission devices.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a plasma enhanced chemical vapordeposition system used to synthesize vertically aligned carbon nanotubes(VACNTs).

FIGS. 2a-2d show low (100×100 μm scan) resolution AFM images of thefollowing samples: an as-received stainless steel (SS) sample, apolished SS sample, a SS sample with temperature ramping (760° C.) in anNH₃ (400 sccm) environment, and a SS sample with temperature ramping(760° C.) in an Ar (400 sccm) environment, respectively. FIGS. 2e-2hshow high (5×5 μm scan) resolution AFM images of the following SSsamples: an as-received SS sample, a polished SS sample, a SS samplewith temperature ramping (760° C.) in an NH₃ (400 sccm) environment, anda SS sample with temperature ramping (760° C.) in an Ar (400 sccm)environment, respectively.

FIG. 3a shows a 1×1 μm scan resolution AFM image of a polished and rampheated sample in an NH₃ environment. FIG. 3b is a plot of catalystparticles sizes (lateral) in nm. FIG. 3c is an SEM image of across-section of a sample heated in a NH₃ environment. FIG. 3d is a plotof an energy dispersive x-ray spectrum (EDS) obtained from a nano-hillshown in FIG. 3 c.

FIGS. 4a, 4b, 4d, and 4e shows SEM image of VACNTs synthesized on SSsubstrate at 650° C., 700° C., 760° C., and 800° C., respectively. FIG.4c is a plot of the variation of yield over different growthtemperatures. FIG. 4f shows a plot of variation of VACNT length overdifferent growth temperatures.

FIG. 5a shows a low-resolution TEM image of as-synthesized VACNTs at760° C. at standard conditions with catalyst particles at the tip. FIG.5b shows a low-resolution TEM image of as-synthesized VACNTs at 760° C.at standard conditions and having a “bamboo-like structure.” FIG. 5c isa plot of the EDS spectrum showing the composition of the particle atthe tip of a VACNT. FIG. 5d is a high resolution TEM (HRTEM) image ofthe particle at the tip of a VACNT enclosed by few layers of graphiticlayers.

FIGS. 6a-6e show SEM images of as-synthesized VACNTs on a SS substrateat growth times of 3 min, 5 min, 10 min, 15 min, and 20 min,respectively. FIG. 6f shows a plot of the variation of VACNT length (μm)with time (min).

FIG. 7a-7d show SEM images of as-synthesized VACNTs grown on SSsubstrate at plasma power of 9 W, 40 W, 70 W and 92 W, respectively.

DETAILED DESCRIPTION

The following disclosure and exemplary embodiments are presented toenable one of ordinary skill in the art to synthesize vertically alignedcarbon nanotube (VACNT) arrays directly on a catalytic conductivesubstrate without addition of an external metallic catalyst layer andwithout any pretreatment to the substrate surface using a plasmaenhanced chemical vapor deposition (PECVD) method, according toembodiments of the subject invention. Various modifications to theembodiments will be readily apparent to those skilled in the art and thegeneric principles herein may be applied to other embodiments. Thus, thedevices and methods related to the VACNT arrays are not intended to belimited to the embodiments shown, but are to be accorded the widestscope consistent with the principles and features described herein.

A catalytic conductive substrate, for example a stainless steelsubstrate, can be loaded on a sample holder inside of a tube of a PECVDdevice and the pressure inside the tube can be adjusted to a basepressure. This catalytic conductive substrate is not subjected topre-treatment in a PECVD device. In addition to stainless steel, thesubstrate can comprise various alloys including, but not limited to, asInconel®, ferronickel, and alnico; and metals including, but not limitedto, iron, nickel, or cobalt. A mixture of carbon source gas at differentflow rates and a plasma etching gas at a constant flow rate can beintroduced to synthesize the VACNTs. In one embodiment, the carbonsource gas is either acetylene (C₂H₂) methane (CH₄), ethylene (C₂H₄), orethanol (C₂H₅OH). In one embodiment, the etching gas can be ammonia gas(NH₃). In addition to any gases described herein, the precursor (carbonsource) gases can comprise ammonia (NH₃) gas and hydrogen (H₂) gas canbe used as reduction gas for assisting the growth of VACNTs. In certainembodiments, the base pressure is set to 0.01 Torr, the carbon sourcegas is introduced to the catalytic conductive substrate at a rate of 10,15, 25, or 35 sccm, and the etching gas is introduced at a rate of 400sccm. The catalytic conductive substrate can be ramp heated to differentgrowth temperatures at a constant rate under the etching gas environmentand at a constant pressure.

In one embodiment, the catalytic conductive substrate can be ramp heatedto different growth temperatures in a range of 650 to 800° C. at aconstant rate of 50° C./min (for example, 650° C., 700° C., 760° C., and800° C.) under the etching gas environment at a constant pressure, forexample 7 Torr.

Once a growth temperature is reached, DC plasma can be initiated andmaintained at different respective plasma power levels in a range of 9to 92 W, for example, 65, 70, 80, and 92 W. Carbon source gas can beintroduced after the plasma becomes stable. The VACNTs can be grown fordifferent time durations between 2 and 20 minutes, for example, 2, 3, 5,10, 15, or 20 minutes, and then allowed to cool down under a basepressure.

A greater understanding of the present invention and of its manyadvantages may be had from the following examples, given by way ofillustration. The following examples are illustrative of some of themethods, applications, embodiments and variants of the presentinvention. They are, of course, not to be considered as limiting theinvention. Numerous changes and modifications can be made with respectto the invention.

VACNT arrays growth experiments were performed on stainless steel 304type (SS-SH-C-6000, thickness 0.63 mm, Kimball Physics, Inc.). Anas-received SS sheet was cut into small pieces having dimensions of 1cm×1 cm×0.63 mm and then polished with sand paper of different gritsizes (Norton 120, 1200, 2400 and 4000 grits, respectively) to a smoothfinish. Then, the pieces were cleaned ultrasonically with acetone andisopropyl alcohol (IPA) baths, each for 10 minutes in order to removeorganic/inorganic contaminants.

Experiments were conducted using a PECVD system 100, as seen in FIG. 1.The PECVD system comprises a tube 110, for example a quartz tube havinga diameter 10 cm and a length of 43 cm, and an external heater 120configured to surround a substrate 130 on a substrate holder. In oneembodiment, the heater 120 is an infrared image furnace. Two metallicrods, for example a cylindrical copper rod (diameter=25.5 mm),respectively having an anode 140 and a cathode 150. A voltage sourceused to apply a bias voltage to create plasma. In one embodiment, theanode 140 and the cathode 150 are at a distance of 1.4 cm from eachother. A DC power supply (not shown) can be connected to the anode 140and the cathode 150. The plasma power between the anode 140 and thecathode 150 can be adjusted to a desired plasma power for a specificapplication. A gas inlet 160 and outlet 170 are present at the twoclosed ends of the tube 110 so that carbon source and etching (orreducing) gases could be fed into and pumped out of the tube 110. Aregulator or multiple regulators (not shown) can be configured tocontrol the flow of gas into and out of the tube 110. A thermocouple 180was used to monitor the temperature and provide feedback to the heater'spower supply. A heat reflector 190 and gas distributor 200 were disposedin the tube 110.

EXAMPLE 1

In order to study the surface morphology prior to the introduction ofthe carbon source gas, two different substrates were prepared by rampheating the substrates to 760° C. (normal ramping heat) under an NH₃environment and an argon (Ar) environment, respectively, both at 400sccm and a pressure of 7 Torr. The study of the surface nano-featuresand composition of the as-prepared samples prior to C₂H₂ introductionwas performed by (a) an atomic force microscopy (AFM) system in tappingmode (Veeco Multimode Nanoscope III D) with a Si tip (spring constant=42N/m and resonance frequency=250-300 KHz) and (b) a dual-beam focused ionbeam (FIB) system equipped with an Energy Dispersive X-ray Spectrometer(EDS) and a Scanning Electron Microscope (SEM).

Microstructural characterization of the as-synthesized VACNTs sampleswas carried out by using a field emission SEM (JEOL JSM-6330F) operatedat an accelerating voltage of 25 KV. FEI Tecnai F30 TEM operated at 300KV equipped with Energy Dispersive Analysis of X-rays (EDAX) technologyand FEI Titan 02 80-200 equipped with Chemi-STEM Technology operated at200 KV were used to study the nanostructures of the as-synthesizedVACNTs. For the TEM experiment, as-synthesized VACNTs were scratched offof the SS substrate with a tweezer, dispersed ultrasonically in alcoholfor 5 min, and transferred to the TEM grid.

The study of surface morphology of the SS substrates prior to theintroduction of carbon source gases provided information regarding thegrowth mechanism of VACNTs on the catalytic substrates. On-sitedecomposition of the carbon source (C₂H₂) and diffusion of the carbonatoms or clusters into the active nucleation sites (nano-sized catalystislands) are part of the VACNT's growth mechanism. Hence, abundant anduniform availability of active nucleation sites on the substrate surfacepromote the synthesis of the VACNTs.

FIGS. 2a and 2e show AFM images of the surfaces of the as-received andpolished SS samples, respectively. Large grains with clear features andhaving a lateral size in a range of a few microns to approximately 17microns were observed on the as-received sample surface. These largegrains on the surface were possibly a thin layer of chromium passiveoxide (Cr₂O₃). Although, having the layer on the surface of the SSsubstrate is advantageous against corrosion, the layer's poisonouseffect on a metal (Fe) catalyst has adverse effect on VACNT growth.Granular microstructures or microscale features can inhibit the VACNTsgrowth, but can be removed through polishing. After polishing, nonano-features, which act as catalytic sites for VACNT growth, on thesurface of the samples were detected (see, for example, FIGS. 2b and 2f).

FIGS. 2c and 2d show AFM images of the sample surfaces ramp heated to atemperature of 760° C. at a rate of 50° C./min under an NH₃ and an Ar(both at 400 sccm) environment, respectively (FIGS. 2g and 2h arerespective corresponding high-resolution AFM images). These AFM imagessuggest that the heat treatment in the presence of these two gasesaffects the surface evolution of the sample substrate in regards toforming the nano-sized catalyst particles. Particularly, the presence ofNH₃ creates a uniform breakup of the surface of the substrates byetching the surface and forming more uniform nano-hills of averagelateral diameter 50.91 nm (see, for example, FIGS. 3a and 3b ). Featureswith a dimension of more than 1 micron were observed in the presence ofthe Ar gas (see, for example, FIG. 2h ).

EXAMPLE 2

To investigate the surface morphology and perform compositional analysisof the nano-features on a substrate heated in the presence of NH₃, across-section of the sample was extracted by using Ga-ion FIB andinvestigated on a FEI Tecnai F30 TEM device operated at 300 KV equippedwith Energy Dispersive Analysis of X-rays (EDAX) technology using STEMimaging.

FIG. 3c shows the cross-sectional structure of the sample in which aplatinum (Pt) layer was deposited to protect the surface features from acurrent beam during the milling process. In addition, a thin layer ofcarbon was deposited prior to the SEM study during the milling process.Beneath this carbon layer, a native surface of the sample wascharacterized by the presence of nano-hills, which were also seen on ahigh-resolution AFM image, (see, for example, FIG. 3a ). FIG. 3d shows aplot of the energy dispersive x-ray spectrum (EDS) obtained from one ofthe nano-hills shown in FIG. 3 c.

The EDS result shows that the constituents of the nano-features compriseFe, Ni, and Cr; which are also the native components of the as-receivedSS. The characteristics of the sample surface, prior to the flow ofcarbon source, contribute to the breakup of the surface of the substrateand are related to the formation of the catalytic active sites used forVACNT nucleation. This surface evolution after polishing and before thesupply of carbon source, as revealed by AFM and STEM images (see, forexample, FIGS. 3a and 3c ), can be attributed to environmental factorssuch as high temperature, etching gas atmosphere, presence of oxides orcarbide, and complex processes related to surface energy. The reductionof metal oxides and carbides to their metal phases is important as theyare not catalytically active for the VACNT synthesis.

The combined effect of these processes results in a chemical andstructural rearrangement at the surface of the SS substrate on an atomicand a nanoscale level. The chemical and structural rearrangement causesthe surface to breakup, which exposes the catalytic active sites. Inaddition, the breakup of the surface results in more surface area andmore crystallographic defects, which generates more catalytically activesites for dissociative adsorption and precipitation of elemental carbonmade available from the dissociation of the carbon source gas at anelevated temperature.

The VACNT growth mechanism on a catalytic substrate can be understood asfollows: preheating the polished SS substrate under a flow of NH₃ gascreates the catalytic active nano-hills on the substrate surface. Thecarbon, released upon the decomposition of the carbon source gas andcoupled with heat from the plasma sheath and heater, dissolves anddiffuses into the catalyst nano-particles. Finally, VACNTs are formed asa result of carbon precipitation from the catalyst surface oncesupersaturation is reached. Each VACNT can contain catalyst particles ata tip depending on the growth mechanism inherited (for example, basegrowth or tip growth).

As VACNTs have highly anisotropic polarizability in the presence ofexternal electric field, the electric field present in the plasma sheathhelps to align the VACNTs (perpendicular to the substrate surface andhereinafter referred to as the vertical direction). Additionally, plasmacan be used to etch the amorphous carbon deposited during the growthprocess. Flow of NH₃ or H₂ (H-rich gas) removes any amorphous carbonthat was not already removed through hydrocarbon precursor dissociation.This etching process can keep the catalyst particles free of amorphouscarbon so that the catalyst gets continuous access to the carbon sourcegas.

FIGS. 4a, 4b, 4d, and 4e show a 15° tilt view of SEM images of VACNTssynthesized at 650° C., 700° C., 760° C., and 800° C. temperature,respectively. These images show the distribution and alignment of theindividual self-standing VACNTs. These samples were prepared by keepingthe other parameters, such as the flow rate of NH₃ (400 sccm), the flowrate of C₂H₂ (15 sccm), the plasma power (70 W), the growth time (10min), and the pressure inside the tube (7 Torr) constant during thesynthesis period.

The substrate was covered by aligned and uniform VACNTs by using PECVDmethod. FIGS. 4a, 4b, 4d, and 4e illustrate the effect of temperature onthe growth of VACNTs on the SS substrate. At a temperature of 650° C.,the sample substrate was non-uniformly covered with VACNTs of short andnon-uniform length, as seen in FIG. 4a . The average diameter of theVACNTs was 101 nm and the length was 2.5 μm. The average length of theVACNTs increased to 4.2 μm and 7.8 μm at temperatures of 700° C. and760° C., respectively. The average diameter was found to increase to114.5 nm and 115 nm at temperatures of 700° C. and 760° C.,respectively. In addition, the sample substrates were covered withVACNTs having a uniform length. The density of the VACNTs wasapproximately 9×108 per cm² for the VACNTs formed at temperatures of700° C. and 760° C. The average length and diameter of the VACNTs were8.1 μm and 105.6 nm respectively for the VACNTs grown at 800° C.

After growing the VACNTs at 800° C. for 10 minutes, the substratesurface was covered with amorphous carbon and some poorly aligned VACNTsas shown in FIG. 4 e. This growth pattern suggests that the growth ofVACNT on a SS substrate happens in a certain temperature range. Atemperature of 650° C. was insufficient to produce the thermal energynecessary for the reconstruction of the substrate surface before theflow of hydrocarbon gas and hence formation of catalytic activenano-hills. The 650° C. temperature is also insufficient for thedecomposition of the hydrocarbon gas (C₂H₂) to liberate the carbon. Alack of carbon and active nucleation sites, where catalytic diffusionand precipitation takes place, leads to a non-uniform growth of VACNTswhich have short and non-uniform lengths.

As the temperature increases above 760° C., the rate of decomposition ofcarbon source gas increased dramatically. The difference between thegrowth rate of VACNTs and decomposition rate of carbon source gas leadto an availability of an excess of carbon which no longer graphitized toform VACNT walls and became attached to the exterior of the VACNTs in anamorphous form, as shown in FIG. 4e . This could have also been causedby ion bombardment as a result of higher ionization of plasma at ahigher temperature.

FIG. 4c shows a plot of the VACNT yields as a function of temperature.As seen in the figure, the yield increases linearly as the temperatureis increased. Amorphous carbon was formed along with the VACNTs grown at800° C. This result is consistent with the results presented by the plotin FIG. 4f , which shows the increase in length with temperature.

Moreover, the average length of VACNTs increased by 0.3 μm whileincreasing the temperature from 760° C. to 800° C. The VACNTssynthesized directly on the catalytic SS substrate at 700° C. had alength 4.2 μm, a diameter of approximately 115 nm, and exhibitedsuperior field emission properties.

The effect of growth time on the morphology of VACNTs is shown in FIG.6. From FIG. 6, it can be seen that the VACNTs grew approximately 1.2 μmin 3 min. The average length increased to 2.2 μm, 7.8 μm, 9.8 μm and10.7 μm when the growth time was increased to 5 min, 10 min, 15 min and,20 min, respectively, and all other conditions were kept constant. Thelength of the VACNTs was found to increase rapidly for the initial 15minutes. However, after 15 minutes, the rate of growth slowed down. Thisis because after certain time, the catalyst particles lose theircatalytic activity and do not allow further dissolution andprecipitation of carbon to form VACNTs during the growth process.

EXAMPLE 3

Paschen's law governs the minimum potential that should be appliedbetween the parallel plate electrodes for a given electrode distance,gas composition, and pressure. Once a sufficient voltage is applied, thebreakdown of the gas composition occurs forming a strong electric field,high ion flux, and plasma near the cathode surface. This plasma enhancesthe VACNTs growth rate and aligns them along the direction of theelectric field. The effect of plasma power on the morphology of theVACNTs is shown in FIG. 7.

Experiments were carried out by keeping all parameters constant with theexception of plasma power. At a lower plasma power (9 W), sparse growthof spaghetti-like VACNTs was observed, as shown in FIG. 7a . As theplasma power was increased, alignment, length and density of the VACNTswere improved. In preferred embodiments, plasma power is approximately70 W during growth and alignment of the VACNTs when using the PECVDsystem as seen in FIG. 1. Short and thick VACNTs having average lengthof approximately 5 μm and diameter of approximately 500 nm can beproduced at a higher plasma power, for example, 92 W, as seen in FIG. 7d.

Low and high resolution TEM images were used to further investigate thenano-structure of the as-synthesized VACNTs. FIGS. 5a and 5b show lowresolution TEM images of the VACNTs (corresponding SEM image shown inFIG. 4d ), which confirm the tubular structure of multi-walled VACNTshaving different diameters. The VACNTs have catalyst particles ofdifferent sizes anchored at the tip, which can also be seen in the SEMimages shown in FIG. 4, indicating the tip growth model.

VACNTs with a smaller diameter contain smaller particles and VACNTs witha large diameter contain larger particles at the tip. Under appropriatesynthesis conditions, VACNTs with a diameter in the range of 5 to 500 nmcan be synthesized. The size of the catalyst nano-hill fragmentsdetached from the surface of the substrate during the growth process cancontrol the diameter of the VACNT. The VACNTs in FIG. 5b displays thebamboo-like structure with a slightly larger diameter at the base thanat the top. This non-uniformity in diameter along the VACNT length canbe due to the change in shape and size of the catalyst particle causedby the dissolution and precipitation of carbon during the growthprocess. The observed bamboo-like structure can be due to the formationof conical (bell-shaped) structures that are doped with nitrogenresulting in excess pentagons at the upper end of the conical structuresand their linear polymerization.

The inset in FIG. 5b shows a high resolution TEM image of thewell-graphitized multilayer of a VACNT wall. The separation between theadjacent layers is approximately 0.34 nm, which corresponds to thelattice fringe distance of (002) graphitic plane showing clearmulti-walled VACNTs. FIG. 5c shows the EDS spectrum obtained from thecatalyst particle present at the tip of the VACNT. This plot shows peakscorrelating to Fe, Cu, and C. Here, the Cu and C peaks are associatedwith TEM sample grid and the VACNTs, respectively. This result suggeststhat the catalyst particle at the tip of the VACNT is Fe. However, EDStaken from other tip particles show the presence of other elements suchas Ni, Mn, Cr, etc. The absence of oxygen in this spectrum rules out thepossibility of formation of Fe-oxides, although presence of C canindicate the presence of Fe carbide.

FIG. 5d shows a high resolution TEM image of a catalyst particle presentat the tip of a VACNT. The top inset is a high magnification image ofthe selected area, as shown in the white box and shows the lattice planedistance of 0.497 nm, which corresponds to the (222) lattice plane ofbody centered cubic (bcc) crystal of Fe. Also, the bottom inset in FIG.5d displays the selected area diffraction (SAED) pattern of the particleat the tip of the VACNT. The bright spots on the SAED image can beindexed as (200), (220) and (222) planes of bcc Fe crystal along the[0-11] zone axis. These results along with the EDS spectrum shown inFIG. 5c confirm the presence of single crystalline Fe particle at thetip of the VACNT. Fe acted as the catalyst during the VACNT synthesisprocess, even though other transition metals such as Ni, Cr, Mn werealso present at the catalyst nano-hills and revealed by the EDS spectrumshown in FIG. 3 d.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

What is claimed is:
 1. A plasma enhanced chemical vapor deposition(PECVD) apparatus, comprising: a tube; two metallic rods disposed insidethe tube and each having a respective electrode disposed on an inner endof each metallic rod of the two metallic rods; a substrate holderdisposed on an inner end of one of the two metallic rods; an externalheating device surrounding the substrate holder; a thermocouple devicedisposed inside the tube and configured to monitor a temperature andprovide feedback to the power supply of the heating device; a gas inletvalve and a gas outlet valve connected to the tube; and a voltage supplyelectrically connected to the two metallic rods and configured to supplya desired bias voltage to generate plasma inside the tube.
 2. Theapparatus according to claim 1, further comprising a heat reflectordisposed inside the tube.
 3. The apparatus according to claim 1, furthercomprising a pump connected to the gas outlet valve.
 4. The apparatusaccording to claim 1, further comprising a pressure gauge configured todetect a pressure inside the tube.
 5. The apparatus according to claim1, the external heating device being an infrared image furnace.
 6. Theapparatus according to claim 1, the tube being a quartz tube.
 7. Theapparatus according to claim 1, further comprising a plurality of gasregulators configured to regulate a flow of gas into and out of thetube.
 8. The apparatus according to claim 1, the two electrodes being ata distance of 1.4 cm from each other.
 9. The apparatus according toclaim 1, the substrate holder being one electrode of the two electrodes.10. The apparatus according to claim 1, further comprising a gasdistributor inside the tube.
 11. A plasma enhanced chemical vapordeposition (PECVD) apparatus, comprising: a tube; a first metallic roddisposed inside the tube and having an anode disposed on an inner endthereof; a second metallic rod disposed inside the tube and having acathode disposed on an inner end thereof; a substrate holder disposed onthe cathode; an external heating device surrounding the substrateholder; a thermocouple device disposed inside the tube and configured tomonitor a temperature and provide feedback to the power supply of theheating device; a gas inlet valve connected to a first end of the tube;a gas outlet valve connected to the tube; and a power supplyelectrically connected to the first metallic rod and the second metallicrod and configured to supply a desired bias voltage to generate plasmainside the tube.
 12. The apparatus according to claim 11, furthercomprising a heat reflector disposed inside the tube.
 13. The apparatusaccording to claim 11, further comprising a gas distributor disposedinside the tube.
 14. The apparatus according to claim 11, furthercomprising a pump connected to the gas outlet valve and a pressure gaugeconfigured to detect a pressure inside the tube.
 15. The apparatusaccording to claim 11, the external heating device being an infraredimage furnace.
 16. The apparatus according to claim 11, the tube being aquartz tube.
 17. The apparatus according to claim 11, further comprisinga plurality of gas regulators configured to regulate a flow of gas intoand out of the tube.
 18. The apparatus according to claim 11, the anodeand the cathode being at a distance of 1.4 cm from each other.
 19. Theapparatus according to claim 11, the anode having a first surface facingthe gas inlet valve and a second surface opposite from the firstsurface, the cathode having a first surface and a second surfaceopposite from the first surface, the first surface of the cathode facinga second end of the tube opposite from the first end of the tube, andthe second surface of the cathode facing the second surface of the anodeand having the substrate holder disposed thereon, the thermocoupledevice being disposed between the first surface of the anode and thefirst end of the tube, and the heat reflector and the gas distributorboth disposed between the thermocouple device and the first end of thetube, the heat reflector and the gas distributor both being disposedcloser to the first end of the tube than they are to the thermocoupledevice.
 20. A plasma enhanced chemical vapor deposition (PECVD)apparatus, comprising: a tube; a first metallic rod disposed inside thetube and having an anode disposed on an inner end thereof; a secondmetallic rod disposed inside the tube and having a cathode disposed onan inner end thereof; a substrate holder disposed on the cathode; anexternal heating device surrounding the substrate holder; a thermocoupledevice disposed inside the tube and configured to monitor a temperatureand provide feedback to the power supply of the heating device; a gasinlet valve connected to a first end of the tube; a gas outlet valveconnected to the tube; a heat reflector disposed inside the tube; a gasdistributor disposed inside the tube; a power supply electricallyconnected to the first metallic rod and the second metallic rod andconfigured to supply a desired bias voltage to generate plasma insidethe tube; a pump connected to the gas outlet valve; a pressure gaugeconfigured to detect a pressure inside the tube; and a plurality of gasregulators configured to regulate a flow of gas into and out of thetube, the external heating device being an infrared image furnace. thetube being a quartz tube, the anode having a first surface facing thegas inlet valve and a second surface opposite from the first surface,the cathode having a first surface and a second surface opposite fromthe first surface, the first surface of the cathode facing a second endof the tube opposite from the first end of the tube, and the secondsurface of the cathode facing the second surface of the anode and havingthe substrate holder disposed thereon, the thermocouple device beingdisposed between the first surface of the anode and the first end of thetube, and the heat reflector and the gas distributor both disposedbetween the thermocouple device and the first end of the tube, the heatreflector and the gas distributor both being disposed closer to thefirst end of the tube than they are to the thermocouple device.